1. Field of the Invention
The present invention relates to a polyamide imide resin composition having excellent heat resistance, mechanical strength, and tracking resistance.
2. Description of the Related Art
In recent years, a material having high heat resistance has been required as an insulating material. Examples of the material having high heat resistance include aromatic polymers such as a polyimide resin and a polyamide imide resin. However, the aromatic polymer has a problem of inferior tracking resistance, which is one of electrical properties required for the insulating material (Kanbe, Hirotaro. (1970): Heat resistance of Polymers, BAIFUKAN CO., LTD.).
Tracking is caused by production and accumulation of free carbon from a solid insulator. For example, a method involving filling a system with an inorganic substance such as a metal hydrate of magnesium hydroxide, aluminum hydroxide, or the like to decrease the free carbon has been proposed as a method for improvement of tracking resistance of a resin. However, the method may impair original properties of the resin such as breaking strength and breaking elongation depending on the filled amount of the inorganic substance. Meanwhile, as a method of releasing carbon to outside the system not as the free carbons but as a volatile carbon compound in thermal decomposition of the resin, for example, a method involving introducing an aliphatic structure to suppress production of the free carbons may be employed. However, in this case, the heat resistance (e.g., a glass transition temperature used as an index of a continuous use temperature) may be lowered. As described above, it is difficult to achieve both the tracking resistance and original properties of the resin.
Meanwhile, in a rotating electrical machine such as a motor or a generator or a transformer, an insulating sheet is disposed between a wire-wound coil and a core or between wire-wound coils where currents with different phases flow to ensure the insulating property. As such insulating sheet, for example, a laminated film obtained by attaching sheets of wholly aromatic polyamide paper to each other via an aromatic polyamide resin-based resin composition has been used (for example, Japanese Patent Application Laid-open No. 2006-321183). In such insulating sheet, the heat resistance is improved by using the aromatic polyamide resin composition between the sheets of wholly aromatic polyamide paper. Such insulating sheet has a laminate structure and hence has a large thickness. Therefore, in order to increase the number of coils to improve its performance, the size inevitably becomes larger, and hence it has been required to decrease the thickness of the insulating sheet.